I have taken the rubber tip off an automotive compression gauge and slipped it over a 1/4" air line and holding against air pressure sufficient to do a leak down test.

I removed that cover. The rack and plungers move freely. All the lifters seem to move the same distance. Noticed number 3 line was seeping from the connection at the pump. Upon inspection I found a chip of paint got between the ferrell and pump. Think that should be good now. It got dark and figured I'd wait for daylight to start it again. No more than was leaking I don't think it should have dropped pressure enough to matter. Could have.  I'm thinking that pump is weak. At a slow idle loosen any of the other lines it about dies and there's a mist of fuel. Do it to number 3 and no change and more of a stream of fuel. 

When I drained the oil from the injector housing I didn't notice any fuel, just moisture. Think I must have had fuel on my hand or something when I checked it the first time. There was quite a bit of moisturized oil behind the inspection cover also. And a bunch of crud. Not sure how it got there  couldn't see any passages there when I cleaned it  Only thing I saw was a slight bit of oil coming around the lifters when running, very slight amount. 

Not that this should matter, but how is your oil level in the pump housing? Did you remove the hex plug and check the level or for fuel? JM

Fuel filters are available from NAPA AUTO. I have a 6 U so I have used these. Napa # 3825, $25 each, Need 4 and the gaskets are shown as a NAPA GAS FILTER # 5635. Why they call a gasket a filter, I have no idea, but they are only a couple of dollars

I looked at my notes and the Napa 3825 cross references to WIX 33825.

On my limited budget, I have to look for the best price. Good Luck. Grant.

I changed the oil. It looked milky . First I thought it smelled like fuel. However after draining it, I think I must have had fuel on my hand. Looked like coffee with a bunch of creamer. Plan to change it again shortly to flush it out a bit. Thinking maybe it wasn't changed for a long time. The starter pinion was the same. Flushed it with diesel fuel. Didn't want to take chance with trying that with the injector cam. The drain for starter was a bit confusing. From looking at the manual, there is no plug where it shows. There is one slightly further back. It is more on the bell housing. Removed it and nothing came out until I shoved a screwdriver in and dug out a bunch of crud. Then what looked like same fluid came out of pinion level plug. Filling pinion housing with diesel fuel made fuel come from that hole. Maybe there's a bad seal? Or maybe it's because it was way overfull. I had level plug in and filled to top. Wanted to clean clutch and brake . But if that's the wrong plug to drain I'm not sure where to drain. There is a plug directly under starter clutch lever. But picture in manual clearly doesn't point to it. I know it's much easier to start now. You had to hold up on pinion lever before. Went to adjust pinion latch. Took out cotter pin on adjusting screw. Turned to other one and was no screw or spring. Made a screw from the end of a 7/16-20 bolt. Found a spring on ebay. Was able to get it all together thru that tiny hole. Was nervous about that. Not easiest thing to not drop screw in there. Not sure about adjustment. Might need a bit tighter. Seemed it could be better if it kicked out just a second or so later. But I've only started it once since fixing it. I'll try a few more times and see.
All the other fluids look okay . Don't know about finals. Don't have anything to remove plugs yet

Think I may have found the problem. I was going to swap 1 and 3 injector pumps to see if the miss moved. Ended up putting them back in the same place. Number 3 uses an o-ring seal. The others use a cone shaped rubber seal. The seal was pretty stuck to the housing. Didn't want to damage it. Also I found there was a red plastic sleeve between the plunger gear and the pump. This kept the plunger from going into the pump all the way. Probably 16th to 8th of an inch. I've got it back together. But #3 injector line won't seal. Just cranking the engine pumps diesel fuel everywhere. The ferrell had a spacer between it and the nut. This is broken with half missing. Therefore, it won't tighten. I've got a new line ordered that I hope will work. It's for a D333. The only one I could find for a D315 was $80 more plus shipping. They appear to have the same connectors.

The cone shaped seals are older. Fat O ring is the replacement. That red sleeve; Do you think someone replaced #3 and forgot to remove the packaging?

I know someone changed one sometime. 1,2,and 4 are the same older type with the cone shape seals and a bleeder screw. #3 has newer oring no bleeder screw, and it's shaped a bit different. And just so happens the new one is on the same cylinder that's not firing. Makes me question if they put that on trying to solve same issue I'm looking at. Maybe gave up and sold it? Thinking that plastic piece may have been there to keep the plunger from bottoming out in the pump when packaged. Saw a new one on eBay that had the same red sleeve on it. I'm a bit anxious to see what it's going to do without that piece on there. Have to wait until at least Friday for the fuel line. Hope the fittings are the same size. Not sure what to think about the fuel gauge. It's saying there's no fuel pressure. It's getting too much fuel for that to be true. It's getting fuel to the gauge. Another gauge says the same. 

Did you go through the filter housing yet? There's a relief valve that gets stuck sometimes or fuel just flows through the passage back to the lift pump or not at all. I had one that was all clogged and had too much pressure and a heck of a time sealing the fuel filter cover. Once cleared, I didn't have any more issues. Perhaps yours is stuck open preventing pressure to build in the fuel rail in the pump housing that feeds the injector pumps. Something else to look at is the actual injector tips. Pull them out hook up a fuel line pointing away in a safe direction, hold a piece of cardboard or something and see if there's a decent spray pattern. Just don't let the fuel stream touch your skin. All that is described in a service manual. I'm not good a posting pictures or I could show you what needs looked at.